I found a cool piece of old equipment. It's a fault voltage circuit interruptor intended to detect ground faults. It's got 2 ground connections, 1 for the incoming ground (auxiliary ground) and 1 for the load ground. If there was a voltage between load ground and auxiliary ground exceeding 50V it tripped. It's a 10x10x10cm box, still wired to a long piece of old NYM cable with the colors black, black, blue, grey and red. Auxiliary ground seems to be a long green wire.